Nitride-based semiconductor device and method for fabricating the same

ABSTRACT

A nitride-based semiconductor light-emitting device  100  includes a GaN substrate  10,  of which the principal surface is an m-plane  12,  a semiconductor multilayer structure  20  that has been formed on the m-plane  12  of the GaN-based substrate  10,  and an electrode  30  arranged on the semiconductor multilayer structure  20.  The electrode  30  includes an Mg alloy layer  32  which is formed of Mg and a metal selected from a group consisting of Pt, Mo, and Pd. The Mg alloy layer  32  is in contact with a surface of a p-type semiconductor region of the semiconductor multilayer structure  20.

TECHNICAL FIELD

The present invention relates to a nitride-based semiconductor deviceand a method for fabricating such a device. More particularly, thepresent invention relates to a GaN-based semiconductor light-emittingdevice such as a light-emitting diode or a laser diode that operates atwavelengths over the ultraviolet range and the entire visible radiationrange, which covers blue, green, orange and white parts of the spectrum.Such a light-emitting device is expected to be applied to various fieldsof technologies including display, illumination and optical informationprocessing in the near future. The present invention also relates to amethod of making an electrode for use in such a nitride-basedsemiconductor device.

BACKGROUND ART

A nitride semiconductor including nitrogen (N) as a Group V element is aprime candidate for a material to make a short-wave light-emittingdevice because its bandgap is sufficiently wide. Among other things,gallium nitride-based compound semiconductors (which will be referred toherein as “GaN-based semiconductors” and which are represented by theformula Al_(x)Ga_(y)In_(z)N (where 0≦x, y, z≦1 and x+y+z=1)) have beenresearched and developed particularly extensively. As a result, bluelight-emitting diodes (LEDs), green LEDs, and semiconductor laser diodesmade of GaN-based semiconductors have already been used in actualproducts (see Patent Documents 1 and 2, for example).

A GaN-based semiconductor has a wurtzite crystal structure. FIG. 1schematically illustrates a unit cell of GaN. In an Al_(x)Ga_(y)In_(z)N(where 0≦x, y, z≦1 and x+y+z=1) semiconductor crystal, some of the Gaatoms shown in FIG. 1 may be replaced with Al and/or In atoms.

FIG. 2 shows four fundamental vectors a₁, a₂, a₃ and c, which aregenerally used to represent planes of a wurtzite crystal structure withfour indices (i.e., hexagonal indices). The fundamental vector c runs inthe [0001] direction, which is called a “c-axis”. A plane thatintersects with the c-axis at right angles is called either a “c-plane”or a “(0001) plane”. It should be noted that the “c-axis” and the“c-plane” are sometimes referred to as “C-axis” and “C-plane”.

In fabricating a semiconductor device using GaN-based semiconductors, ac-plane substrate, i.e., a substrate of which the principal surface is a(0001) plane, is used as a substrate on which GaN semiconductor crystalswill be grown. In a c-plane, however, there is a slight shift in thec-axis direction between a Ga atom layer and a nitrogen atom layer, thusproducing electrical polarization there. That is why the c-plane is alsocalled a “polar plane”. As a result of the electrical polarization, apiezoelectric field is generated in the InGaN quantum well of the activelayer in the c-axis direction. Once such a piezoelectric field has beengenerated in the active layer, some positional deviation occurs in thedistributions of electrons and holes in the active layer. Consequently,the internal quantum yield decreases, thus increasing the thresholdcurrent in a semiconductor laser diode and increasing the powerdissipation and decreasing the luminous efficacy in an LED. Meanwhile,as the density of injected carriers increases, the piezoelectric fieldis screened, thus varying the emission wavelength, too.

Thus, to overcome these problems, it has been proposed that a substrateof which the principal surface is a non-polar plane such as a (10-10)plane that is perpendicular to the [10-10] direction and that is calledan “m-plane” (m-plane GaN-based substrate) be used. As used herein, “−”attached on the left-hand side of a Miller-Bravais index in theparentheses means a “bar” (a negative direction index). The “m-plane” issometimes expressed as “M-plane”. As shown in FIG. 2, the m-plane isparallel to the c-axis (i.e., the fundamental vector c) and intersectswith the c-plane at right angles. On the m-plane, Ga atoms and nitrogenatoms are on the same atomic-plane. For that reason, no electricalpolarization will be produced perpendicularly to the m-plane. That iswhy if a semiconductor multilayer structure is formed perpendicularly tothe m-plane, no piezoelectric field will be generated in the activelayer, thus overcoming the problems described above. The “m-plane” is ageneric term that collectively refers to a family of planes including(10-10), (−1010), (1-100), (−1100), (01-10) and (0-110) planes.

Also, as used herein, the “X-plane growth” means epitaxial growth thatis produced perpendicularly to the X plane (where X=c or m) of ahexagonal wurtzite structure. As for the X-plane growth, the X planewill be sometimes referred to herein as a “growing plane”. A layer ofsemiconductor crystals that have been formed as a result of the X-planegrowth will be sometimes referred to herein as an “X-plane semiconductorlayer”.

CITATION LIST Patent Literature

Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-308462

Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-332697

Patent Document 3: Japanese Laid-Open Patent Publication No. 8-64871

Patent Document 4: Japanese Laid-Open Patent Publication No. 11-40846

SUMMARY OF INVENTION Technical Problem

As described above, a GaN-based semiconductor device that has been grownon an m-plane substrate would achieve far more beneficial effects thanwhat has been grown on a c-plane substrate but still has the followingdrawback. Specifically, a GaN-based semiconductor device that has beengrown on an m-plane substrate has higher contact resistance than whathas been grown on a c-plane substrate, which constitutes a serioustechnical obstacle to using such a GaN-based semiconductor device thathas been grown on an m-plane substrate.

Under the circumstances such as these, the present inventorswholeheartedly carried out extensive research to overcome such a problemwith the prior art that a GaN-based semiconductor device, grown on anm-plane as a non-polar plane, would have high contact resistance. As aresult, we found an effective means for reducing the contact resistance.

It is therefore an object of the present invention to provide astructure and manufacturing process that will be able to reduce thecontact resistance of a GaN-based semiconductor device that has beenfabricated by producing a crystal growth on an m-plane substrate.

Solution to Problem

The first nitride-based semiconductor device of the present inventionincludes: a nitride-based semiconductor multilayer structure including ap-type semiconductor region, a surface of the p-type semiconductorregion being an m-plane; and an electrode that is arranged on the p-typesemiconductor region, wherein the p-type semiconductor region is made ofan Al_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg alloy layer which is in contact withthe surface of the p-type semiconductor region and which is made of Mgand a metal selected from a group consisting of Pt, Mo, and Pd.

In one embodiment, the electrode includes the Mg alloy layer and a metallayer provided on the Mg alloy layer, and the metal layer is formed of ametal which is selected from Pt, Mo, and Pd and which is contained inthe Mg alloy layer.

In one embodiment, the semiconductor multilayer structure includes anactive layer which includes an Al_(a)In_(b)Ga_(c)N layer (where a+b+c=1,a≧0, b≧0 and c≧0), the active layer being configured to emit light.

In one embodiment, the p-type semiconductor region is a p-type contactlayer.

In one embodiment, the Mg alloy layer has a thickness of 0.1 nm to 5 nm.

In one embodiment, the thickness of the Mg alloy layer is equal to orsmaller than that of the Pt layer.

In one embodiment, in the Mg alloy layer, a concentration of N is lowerthan a concentration of Ga.

In one embodiment, the nitride-based semiconductor device furtherincludes a semiconductor substrate that supports the semiconductormultilayer structure.

A light source of the present invention includes: a nitride-basedsemiconductor light-emitting device; and a wavelength converterincluding a phosphor that converts a wavelength of light emitted fromthe nitride-based semiconductor light-emitting device, wherein thenitride-based semiconductor light-emitting device includes anitride-based semiconductor multilayer structure including a p-typesemiconductor region, a surface of the p-type semiconductor region beingan m-plane, and an electrode that is arranged on the p-typesemiconductor region, the p-type semiconductor region is made of anAl_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg alloy layer which is in contact withthe surface of the p-type semiconductor region and which is made of Mgand a metal selected from a group consisting of Pt, Mo, and Pd.

A nitride-based semiconductor device fabrication method of the presentinvention includes the steps of: (a) providing a substrate; (b) formingon the substrate a nitride-based semiconductor multilayer structureincluding a p-type semiconductor region, a surface of the p-typesemiconductor region being an m-plane; and (c) forming an electrode onthe surface of the p-type semiconductor region of the semiconductormultilayer structure, wherein step (c) includes forming an Mg alloylayer on the surface of the p-type semiconductor region, the Mg alloylayer being made of Mg and a metal selected from a group consisting ofPt, Mo, and Pd.

In one embodiment, the step of forming the Mg alloy layer includesforming an Mg layer on the surface of the p-type semiconductor region,forming on the Mg layer a conductive layer selected from a groupconsisting of Pt, Mo, and Pd, and performing a heat treatment to alloythe Mg layer and at least part of the conductive layer.

In one embodiment, the heat treatment is performed at a temperature of500° C. to 700° C.

In one embodiment, the heat treatment is performed at a temperature of550° C. to 650° C.

In one embodiment, the step of forming the Mg layer includes irradiatingMg with pulses of an electron beam such that Mg is deposited onto thesurface of the p-type semiconductor region.

In one embodiment, the Mg layer is deposited on the semiconductormultilayer structure so as to have a thickness of 0.1 nm to 5 nm.

In one embodiment, the method further includes removing the substrateafter step (b).

In one embodiment, the step of forming the Mg alloy layer includesdepositing a mixture or compound of Mg and a metal selected from a groupconsisting of Pt, Mo, and Pd onto the surface of the p-typesemiconductor region by means of evaporation, and performing a heattreatment.

The second nitride-based semiconductor device of the present inventionincludes: a nitride-based semiconductor multilayer structure including ap-type semiconductor region, a surface of the p-type semiconductorregion being an en-plane; and an electrode that is arranged on thep-type semiconductor region, wherein the p-type semiconductor region ismade of an Al_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0,and z≧0), and the electrode includes an Mg alloy island provided on thesurface of the p-type semiconductor region, the Mg alloy island beingmade of Mg and a metal selected from a group consisting of Pt, Mo, andPd.

In one embodiment, the electrode includes the Mg alloy island and ametal layer provided on the Mg alloy island, and the metal layer isformed of a metal which is selected from Pt, Mo, and Pd and which iscontained in the Mg alloy island.

Advantageous Effects of Invention

In a nitride-based semiconductor device according to the presentinvention, an electrode on a semiconductor multilayer structure includesan Mg alloy layer that is in contact with the surface (which is anm-plane) of a p-type impurity region. As a result, the contactresistance can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view schematically illustrating a unit cell ofGaN.

FIG. 2 is a perspective view showing four fundamental vectors a₁, a₂, a₃and c representing a wurtzite crystal structure.

FIG. 3( a) is a schematic cross-sectional view illustrating anitride-based semiconductor light-emitting device 100 as a preferredembodiment of the present invention, and FIGS. 3( b) and 3(c) illustratethe crystal structures of an m-plane and a c-plane, respectively.

FIG. 4A shows the current-voltage characteristic under the conditionthat two Pd/Pt electrodes are in contact with a p-type GaN layer.

FIG. 4B shows the current-voltage characteristic under the conditionthat two Mg alloy layer electrodes are in contact with a p-type GaNlayer.

FIG. 4C is a graph which shows the specific contact resistances (Ω·cm²)of the devices in which the above-described Pd/Pt electrode and Mg—Ptalloy/Pt electrode are respectively used.

FIG. 4D is a pattern diagram of a TLM electrode.

FIG. 5 is a graph which shows the dependence of the contact resistanceon the heat treatment temperature.

FIG. 6 shows a profile of Ga in the depth direction in an electrodestructure (Mg/Pt) which was obtained by SIMS analysis.

FIG. 7 shows a profile of N in the depth direction in an electrodestructure (Mg/Pt) which was obtained by SIMS analysis.

FIG. 8( a) shows the current-voltage characteristics of light-emittingdiodes that respectively use an electrode consisting of Mg—Pt alloy/Ptlayers, an electrode consisting of Mg/Pt layers, and an electrodeconsisting of Pt/Pd layers. FIG. 8( b) is a graph showing the contactresistances of the light-emitting diodes.

FIGS. 9( a) and 9(b) are light microscopic photographs of a surface ofan electrode consisting of Mg—Pt alloy/Pt layers and a surface of anelectrode consisting of Mg/Pt layers, which are presented as substitutesfor drawings.

FIG. 10( a) is a graph showing the contact resistances of a device inwhich an electrode consisting of an Au layer is used and a device inwhich an electrode consisting of Mg—Au alloy/Au layers is used. FIGS.10( b) and 10(c) are light microscopic photographs of a surface of anelectrode consisting of Mg—Au alloy/Au layers and a surface of anelectrode consisting of an Au layer, which are presented as substitutesfor drawings.

FIG. 11 is a cross-sectional view illustrating a preferred embodiment ofa white light source.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. In the drawings,any elements shown in multiple drawings and having substantially thesame function will be identified by the same reference numeral for thesake of simplicity. It should be noted, however, that the presentinvention is in no way limited to the specific preferred embodiments tobe described below.

FIG. 3( a) schematically illustrates the cross-sectional structure of anitride-based semiconductor light-emitting device 100 as a preferredembodiment of the present invention. What is illustrated in FIG. 3( a)is a semiconductor device made of GaN semiconductors and has anitride-based semiconductor multilayer structure.

The nitride-based semiconductor light-emitting device 100 of thispreferred embodiment includes a GaN-based substrate 10, of which theprincipal surface 12 is an m-plane, a semiconductor multilayer structure20 that has been formed on the GaN-based substrate 10, and an electrode30 arranged on the semiconductor multilayer structure 20. In thispreferred embodiment, the semiconductor multilayer structure 20 is anm-plane semiconductor multilayer structure that has been formed throughan m-plane crystal growth and its principal surface is an m-plane. Itshould be noted, however, that a-plane GaN could grow on an r-planesapphire substrate in some instances. That is why according to thegrowth conditions, the principal surface of the GaN-based substrate 10does not always have to be an m-plane. In the semiconductor multilayerstructure 20 of the present invention, at least the surface of itsp-type semiconductor region that is in contact with an electrode needsto be an m-plane.

The nitride-based semiconductor light-emitting device 100 of thispreferred embodiment includes the GaN-based substrate 10 to support thesemiconductor multilayer structure 20. However, the device 100 may haveany other substrate instead of the GaN-based substrate 10 and could alsobe used without the substrate.

FIG. 3( b) schematically illustrates the crystal structure of anitride-based semiconductor, of which the principal surface is anm-plane, as viewed on a cross section thereof that intersects with theprincipal surface of the substrate at right angles. Since Ga atoms andnitrogen atoms are present on the same atomic-plane that is parallel tothe m-plane, no electrical polarization will be produced perpendicularlyto the m-plane. That is to say, the m-plane is a non-polar plane and nopiezoelectric field will be produced in an active layer that growsperpendicularly to the m-plane. It should be noted that In and Al atomsthat have been added will be located at Ga sites and will replace the Gaatoms. Even if at least some of the Ga atoms are replaced with those Inor Al atoms, no electrical polarization will still be producedperpendicularly to the m-plane.

Such a GaN-based substrate, of which the principal surface is anm-plane, will be referred to herein as an “m-plane GaN-based substrate”.To obtain a nitride-based semiconductor multilayer structure that hasgrown perpendicularly to the m-plane, typically such an m-plane GaNsubstrate may be used and semiconductors may be grown on the m-plane ofthat substrate. However, the principal surface of the substrate does nothave to be an m-plane as described above, and the device as a finalproduct could already have its substrate removed.

The crystal structure of a nitride-based semiconductor, of which theprincipal surface is a c-plane, as viewed on a cross section thereofthat intersects with the principal surface of the substrate at rightangles is illustrated schematically in FIG. 3( c) just for a reference.In this case, Ga atoms and nitrogen atoms are not present on the sameatomic-plane, and therefore, electrical polarization will be producedperpendicularly to the c-plane. Such a GaN-based substrate, of which theprincipal surface is a c-plane, will be referred to herein as a “c-planeGaN-based substrate”.

A c-plane GaN-based substrate is generally used to grow GaN-basedsemiconductor crystals thereon. In such a substrate, a Ga (or In) atomlayer and a nitrogen atom layer that extend parallel to the c-plane areslightly misaligned from each other in the c-axis direction, andtherefore, electrical polarization will be produced in the c-axisdirection.

Referring to FIG. 3( a) again, on the principal surface (that is anm-plane) 12 of the m-plane GaN-based substrate 10, the semiconductormultilayer structure 20 is formed. The semiconductor multilayerstructure 20 includes an active layer 24 including anAl_(a)In_(b)Ga_(c)N layer (where a+b+c=1, a≧0, b≧0 and c≧0), and anAl_(d)Ga_(e)N layer (where d+e=1, d≧0 and e≧0) 26, which is located onthe other side of the active layer 24 opposite to the m-plane 12. Inthis embodiment, the active layer 24 is an electron injection region ofthe nitride-based semiconductor light-emitting device 100.

The semiconductor multilayer structure 20 of this preferred embodimenthas other layers, one of which is an Al_(u)Ga_(v)In_(w)N layer (whereu+v+w=1, u≧0, v≧0 and w≧0) 22 that is arranged between the active layer24 and the substrate 10. The Al_(u)Ga_(v)In_(w)N layer 22 of thispreferred embodiment has first conductivity type, which may be n-type,for example. Optionally, an undoped GaN layer could be inserted betweenthe active layer 24 and the Al_(d)Ga_(e)N layer 26.

In the Al_(d)Ga_(e)N layer 26, the mole fraction d of Al does not haveto be uniform, but could vary either continuously or stepwise, in thethickness direction. In other words, the Al_(d)Ga_(e)N layer 26 couldhave a multilayer structure in which a number of layers with mutuallydifferent Al mole fractions d are stacked one upon the other, or couldhave its dopant concentration varied in the thickness direction. Toreduce the contact resistance, the uppermost portion of theAl_(d)Ga_(e)N layer 26 (i.e., the upper surface region of thesemiconductor multilayer structure 20) is preferably a layer that has anAl mole fraction d of zero (i.e., a GaN layer).

An electrode 30 has been formed on the semiconductor multilayerstructure 20. The electrode 30 of this embodiment may be an electrodeincluding an Mg alloy layer 32 that is made of Pt and Mg. Provided onthe Mg alloy layer 32 is a metal layer 34 made of Pt. Here, “Mg alloylayer” refers to an Mg layer in which a metal, such as Pt, is containedat a concentration of at least a few percent (which may be equal to orgreater than one percent). In the present embodiment, the Mg alloy layer32 contains Pt, which is a metal that is a constituent of the metallayer 34, at a concentration of at least a few percent.

In the electrode 30, the Mg alloy layer 32 is in contact with the p-typesemiconductor region of the semiconductor multilayer structure 20 andfunctions as a portion of a p-(or p-side) electrode. In this preferredembodiment, the Mg alloy layer 32 is in contact with the Al_(d)Ga_(e)Nlayer 26 that is doped with a dopant of a second conductivity type(e.g., p-type), which may be Mg. Examples of other preferred p-typedopants include Zn and Be.

The metal layer 34 that is in contact with the surface of the Mg alloylayer 32 is not limited to a Pt layer but may also be a layer of a metalthat would make an alloy with Mg less easily than Au (gold). In otherwords, at least one type of metal selected from the group consisting ofPt, Mo, and Pd may be used. As the material of the metal layer 34 thatis in contact with the surface of the Mg alloy layer 32, Au is notpreferred because it would readily be alloyed with Mg. Pt, Mo, and Pdare metals that are less likely to be alloyed with Mg than Au is but mayreact with part of Mg to form an alloy layer by means of a heattreatment which will be described later.

The Mg alloy layer 32 is preferably formed by depositing a metal layerof, for example, Pt, on an Mg layer and thereafter performing a heattreatment on the resultant structure. If the metal layer depositedbefore the heat treatment is relatively thick, there is a remainingmetal layer 34 on the Mg alloy layer 32 formed by the heat treatment. Onthe other hand, if the metal layer deposited before the heat treatmentis relatively thin, the metal layer 34 may be entirely alloyed with Mgby the heat treatment. In this case, the electrode 30 is constitutedonly of the Mg alloy layer 32.

Note that the Mg alloy layer 32 may be formed by performing a vapordeposition using a mixture or compound of a metal that is to be aconstituent of the metal layer 34 and Mg as a source material andthereafter performing a heat treatment on the resultant structure. Inthis case, immediately after the Mg alloy layer 32 has been deposited,the metal layer 34 does not exist on the Mg alloy layer 32. Thereafter,the metal layer 34 may not be deposited on the Mg alloy layer 32, sothat the electrode 30 is constituted only of the Mg alloy layer 32.Alternatively, the metal layer 34 may be deposited on the Mg alloy layer32 when necessary.

At least part of the Mg alloy layer 32 may undergo aggregation to formislands due to a heat treatment performed after the deposition, so thatthe islands are separated from one another with spaces. In this case, Ptatoms that constitute the metal layer 34 intervene between therespective islands of the Mg alloy. At least part of the metal layer 34may undergo aggregation to form islands.

On any of the above-described electrode, together with theabove-described metal layer or alloy layer, any electrode layer or wirelayer made of a different metal or alloy may be provided.

The thickness of the electrode 30 of the present embodiment is, forexample, 1 nm to 200 nm. In the case where the metal layer 34 isprovided on the Mg alloy layer 32, the thickness of the Mg alloy layer32 is smaller than that of the metal layer 34. In this case, thethickness of the Mg alloy layer 32 is for example 5 nm or less(preferably, from 0.1 nm to 5 nm). When the thickness of the Mg layerdeposited before the heat treatment is greater than nm, part of the Mglayer is not alloyed by the heat treatment, so that there may be aremaining Mg layer between the Mg alloy layer 32 and the Al_(d)Ga_(e)Nlayer 26. This is because the metal that constitutes the metal layer 34,such as Pt, is less likely to be alloyed with Mg. If there is aremaining Mg layer, the adhesiveness with the underlying semiconductormultilayer structure 20 may be relatively low. In view of such, thethickness of the Mg layer deposited before the heat treatment ispreferably 5 nm or less. The thickness of the Mg alloy layer 32 formedby the heat treatment is preferably 5 nm or less.

The thickness of the metal layer (e.g., Pt layer) 34 provided over theMg alloy layer 32 is, for example, 200 nm or less (preferably, from 1 nmto 200 nm). The reason why the thickness of the Mg alloy layer 32 issmaller than that of the metal layer 34 is to prevent separation of theMg alloy layer 32 and the Al_(d)Ga_(e)N layer 26 which would be causeddue to disturbed balance of strain between the Mg alloy layer 32 and themetal layer 34. The metal layer 34 significantly contributes to, forexample, prevention of oxidation of the Mg alloy layer 32, but is notabsolutely indispensable.

Meanwhile, the GaN-based substrate 10, of which the principal surface 12is an m-plane, may have a thickness of 100 μm to 400 μm, for example.This is because if the wafer has a thickness of at least approximately100 μm, then there will be no trouble handling such a wafer. It shouldbe noted that as long as the substrate 10 of this preferred embodimenthas an m-plane principal surface 12 made of a GaN-based material, thesubstrate 10 could have a multilayer structure. That is to say, theGaN-based substrate 10 of this preferred embodiment could also refer toa substrate, at least the principal surface 12 of which is an m-plane.That is why the entire substrate could be made of a GaN-based material.Or the substrate may also be made of the GaN-based material and anothermaterial in any combination.

In the structure of this preferred embodiment, an electrode 40 has beenformed as an n-side electrode on a portion of an n-typeAl_(u)Ga_(v)In_(w)N layer 22 (with a thickness of 0.2 μm to 2 μm, forexample) which is located on the substrate 10. In the exampleillustrated in FIG. 3( a), in the region of the semiconductor multilayerstructure 20 where the electrode 40 is arranged, a recess 42 has beencut so as to expose a portion of the n-type Al_(u)Ga_(v)In_(w)N layer22. And the electrode has been formed on the exposed surface of then-type Al_(u)Ga_(v)In_(w)N layer 22 at the bottom of the recess 42. Theelectrode 40 may have a multilayer structure consisting of Ti, Al and Tilayers and may have a thickness of 100 nm to 200 nm, for example.

In this preferred embodiment, the active layer 24 has a GaInN/GaNmulti-quantum well (MQW) structure (with a thickness of 81 nm, forexample) in which Ga_(0.9)In_(0.1)N well layers (each having a thicknessof 9 nm, for example) and GaN barrier layers (each having a thickness of9 nm, for example) are alternately stacked one upon the other.

On the active layer 24, stacked is the p-type Al_(d)Ga_(e)N layer 26,which may have a thickness of 0.2 μm to 2 μm. Optionally, an undoped GaNlayer could be inserted between the active layer 24 and theAl_(d)Ga_(e)N layer 26 as described above.

In addition, a GaN layer of the second conductivity type (which may bep-type, for example) could be formed on the Al_(d)Ga_(e)N layer 26.Furthermore, a contact layer of p⁺-GaN and the Mg alloy layer 32 couldbe stacked in this order on that GaN layer. In that case, the GaNcontact layer could also be regarded as forming part of theAl_(d)Ga_(e)N layer 26, not a layer that has been stacked separatelyfrom the Al_(d)Ga_(e)N layer 26.

FIG. 4A shows the current-voltage characteristic under the conditionthat two Pd/Pt electrodes are in contact with a p-type GaN layer. FIG.4B shows the current-voltage characteristic under the condition that twoMg alloy layer electrodes are in contact with a p-type GaN layer. ThePd/Pt electrode used herein was an electrode (m-plane GaN (Pd/Pt))formed by sequentially forming a Pd layer and a Pt layer in this orderon a p-type m-plane GaN layer and thereafter performing a heat treatmenton the resultant structure in a nitrogen atmosphere. The Mg alloy layerelectrode used herein was an electrode (m-plane GaN (Mg—Pt alloy/Pt))formed by sequentially depositing an Mg layer and a Pt layer in thisorder by means of evaporation on a p-type m-plane GaN layer andthereafter performing a heat treatment on the resultant structure in anitrogen atmosphere such that Mg and Pt are alloyed. The structures andheat treatment conditions of these electrodes are shown below in TABLE1.

TABLE 1 Heat treatment Plane Thickness (before temperature orientationp-electrode heat treatment) and duration m-plane Pd/Pt Pd 40 nm/Pt 35 nm500° C., 10 min. m-plane Mg—Pt Alloy/Pt Mg 2 nm/Pt 75 nm 600° C., 10min.

In the present embodiment, the heat treatment described in TABLE 1 isperformed such that the Mg layer, which is in contact with the p-typeGaN layer, and part of the Pt layer (a side of the Pt layer which is incontact with the Mg layer) are alloyed. Further, the Mg alloy layer isheated while it is in contact with the p-type GaN layer, whereby anexcellent Mg alloy layer electrode (Mg—Pt alloy/Pt electrode) can beformed.

The curves of the current-voltage characteristic shown in FIGS. 4A and4B respectively correspond to the distances between electrodes of theTLM (Transmission Line Method) electrode pattern shown in FIG. 4D. FIG.4D shows an arrangement of a plurality of electrodes of 100 μm×200 μmwith the intervals of 8 μm, 12 μm, 16 μm, and 20 μm.

FIG. 4C is a graph which shows the specific contact resistances (Ω·cm²)of the devices in which the above-described Pd/Pt electrode and Mg—Ptalloy/Pt electrode were respectively used. The contact resistance wasevaluated using the TLM. Referring to the ordinate axis, “1.0E-01” means“1.0×10⁻¹”, and “1.0E-02” means “1.0×10⁻²”. Hence, “1.0E+X” means“1.0×10^(X)”.

Pd is a metal of a large work function, which has been conventionallyused for the p-electrode. In the Pd/Pt electrode, Pd is in contact withthe p-type GaN layer. The graph of FIG. 4A (the current-voltagecharacteristic of the Pd/Pt electrode) shows a Schottky-type non-ohmiccharacteristic (Schottky voltage: about 2 V). On the other hand, noSchottky voltage is seen in the graph of FIG. 4B (the current-voltagecharacteristic of the Mg alloy layer electrode). Thus, it can beunderstood that this Mg alloy layer electrode substantially forms anohmic contact with the p-type GaN layer. Disappearance of the Schottkyvoltage is critical in decreasing the operating voltages of devices,such as light-emitting diodes, laser diodes, etc.

Further, as shown in FIG. 4C, the Mg—Pt alloy/Pt electrode exhibits alower specific contact resistance (Ω·cm²) than the Pd/Pt electrode byapproximately one order of magnitude. The present embodimentsuccessfully provides marvelous effects which would not be achieved bythe conventional approach of using a metal of a large work function forthe p-electrode.

Note that, when the Mg/Pt electrode is in contact with the c-planep-type GaN layer, the contact resistance obtained is slightly lower thana case where the Pd/Pt electrode is used. When the contact surface is anm-plane, the Mg/Pt electrode exhibits a considerably lower contactresistance than the Pd/Pt electrode (see Japanese Patent Application No.2009-536554). It is estimated that the present invention that uses theMg—Pt alloy/Pt electrode would provide a similar result.

Next, the dependence of the contact resistance on the heat treatmenttemperature is described.

FIG. 5 shows the result of an electrode formed by sequentiallydepositing an Mg layer and a Pt layer in this order by means ofevaporation on the m-plane of a p-type GaN layer and thereafterperforming a heat treatment on the resultant structure in a nitrogenatmosphere such that Mg and Pt are alloyed (i.e., m-plane GaN (Mg—Ptalloy/Pt)). FIG. 5 also shows, for comparison purposes, the result of anelectrode formed by sequentially forming a Pd layer and a Pt layer inthis order on a p-type m-plane GaN layer and thereafter performing aheat treatment on the resultant structure in a nitrogen atmosphere(m-plane GaN (Pd/Pt)).

The data shown in FIG. 5 were obtained from samples in which the Mglayer was deposited using a pulse evaporation process. The pulseevaporation process will be described later. In the experimentalexamples of the present invention which are described in the presentspecification, the Mg layer was deposited by the pulse evaporationprocess. Metals other than Mg (e.g., Pd, Pt, Au) were deposited using acommon electron beam evaporation process.

The Mg—Pt alloy/Pt electrode and the Pd/Pt electrode are both in contactwith the Mg-doped m-plane GaN layer. The m-plane GaN layer that is incontact with these electrodes is doped with 7×10¹⁹ cm⁻³ Mg to a depth of20 nm as measured from the surface (i.e., the uppermost surface regionwith a thickness of 20 nm). On the other hand, the rest of the m-planeGaN layer, of which the depth exceeds 20 nm as measured from thesurface, is doped with 1×10¹⁹ cm⁻³ Mg. If the concentration of thep-type dopant is locally increased in this manner in the uppermostsurface region of the GaN layer that is in contact with the p-electrode,the contact resistance can be reduced to the lowest possible level. Ontop of that, by adopting such a doping scheme, the in-planenon-uniformity of the current-voltage characteristic can also bereduced. As a result, the variation in drive voltage between respectivechips can also be reduced. That is why in every experimental exampledisclosed in this application, the surface region of the p-type GaNlayer that is in contact with the electrode is doped with 7×10¹⁹ cm⁻³ Mgto a depth of 20 nm as measured from the surface, while the other deeperregion is doped with 1×10¹⁹ cm⁻³ Mg.

The thicknesses of the respective layers before the heat treatment areshown below in TABLE 2.

TABLE 2 Plane Thickness (before orientation p-electrode heat treatment)m-plane Mg—Pt Alloy/Pt Mg 2 nm/Pt 75 nm m-plane Pd/Pt Pd 40 nm/Pt 35 nm

First, in the case of the Pd/Pt electrode, the contact resistancescarcely changed even after the heat treatment at 500° C. When the heattreatment temperature exceeded 500° C., the contact resistanceincreased.

On the other hand, in the case of the Mg—Pt alloy/Pt electrode, when theheat treatment temperature was 500° C. or higher, the contact resistancesharply decreased. In the present embodiment, it is the Mg layer that isin contact with the p-type m-plane GaN layer before the heat treatment.However, the heat treatment at 500° C. or higher causes the Mg layer tobe alloyed with the Pt layer, so that it is the Mg alloy layer that isin contact with the p-type m-plane GaN layer after the heat treatment.It is seen from FIG. 5 that, in the case of the m-plane GaN (Mg—Ptalloy/Pt) electrode, when the heat treatment temperature was 600° C.,the contact resistance further decreased. When the heat treatment wasperformed at a further increased temperature, 700° C., the contactresistance was higher than that obtained when the heat treatmenttemperature was 600° C. but was smaller than the contact resistanceobtained in the case of the conventional m-plane GaN (Pd/Pt) electrode.

Therefore, the heat treatment temperature for the Mg—Pt alloy/Ptelectrode is preferably 500° C. or higher, for example. The upper limitof the heat treatment temperature is preferably 700° C. or less because,if it exceeds 700° C. to reach a predetermined temperature (e.g., 800°C.) or higher, deterioration in the film quality of the electrode andthe GaN layer would increase. In addition, more preferably, thetemperature range is from 550° C. to 650° C., in which the contactresistance further decreases.

FIG. 6 shows the profile of Ga in the depth direction in the electrodestructure (Mg—Pt alloy/Pt) which was obtained using a SIMS. Before theheat treatment, the thickness of the Mg layer was 2 nm, and thethickness of the Pt layer was 75 nm. The heat treatment produced the Mgalloy layer which has a thickness of 2 nm. The heat treatment wasperformed at 600° C. in a nitrogen atmosphere for 10 minutes. Theordinate axis of the graph represents the signal intensity of the SIMSdetector, which is proportional to the atomic concentration. In theabscissa axis of FIG. 6, Distance 0 μm approximately corresponds to theposition of the interface between the p-type GaN layer and the Mg alloylayer. Note that the origin of the abscissa axis (0 μm) is adjusted tobe coincident with the position of a Ga peak. As for the abscissa axis,the negative (−) value range is on the electrode side, and the positive(+) value range is on the p-type GaN side. The ordinate axis isnormalized with the Ga concentration in as-deposited GaN crystals(before the heat treatment) being arbitrarily assumed as 1. Calculatingfrom the atomic density of the base, the intensity of 1×10⁻³ on theordinate axis approximately corresponds to the concentration of1×10¹⁹cm⁻³.

As shown in FIG. 6, the Ga concentration in the Mg alloy layer after theheat treatment was higher than before the heat treatment. It isunderstood from this result that Ga was diffused into the Mg alloy layerby the heat treatment. Further, the sample that underwent the heattreatment at 500° C. or higher exhibited a decreased contact resistance,and hence, it was confirmed that there is some correlation between theamount of Ga diffused into the Mg alloy layer and the contactresistance, although the cause of the decrease in contact resistance isnot specifically identified. It was also confirmed that, in the samplethat exhibited the lowest contact resistance, the Ga concentration inthe Mg alloy layer was 10¹⁹ cm⁻³ or higher.

FIG. 7 shows the profile of nitrogen atoms in the depth direction in theelectrode structure (Mg—Pt alloy/Pt) which was obtained using a SIMS.Before the heat treatment, the thickness of the Mg layer was 2 nm, andthe thickness of the Pt layer was 75 nm. The heat treatment produced theMg alloy layer which has a thickness of 2 nm. The heat treatment wasperformed at 600° C. in a nitrogen atmosphere for minutes. In the graphof FIG. 7, the ordinate axis represents the N intensity, and theabscissa axis represents the distance in the depth direction. The Nintensity of 1×10⁻³ substantially corresponds to an N concentration of1×10¹⁹ cm⁻³. In the abscissa axis, the negative (−) value range is onthe electrode side, and the positive (+) value range is on the p-typeGaN side. The ordinate axis is normalized with the N concentration inas-deposited GaN crystals (before the heat treatment) being arbitrarilyassumed as 1. The origin of the abscissa axis (0 μm) approximatelycorresponds to the position of the interface between the p-type GaNlayer and the Mg layer. As apparent from FIG. 7, even in the electrodestructure after the heat treatment, diffusion of N into the Mg alloylayer was not confirmed.

As described above, the present inventor found that, when the heattreatment (at 600° C. in a nitrogen atmosphere for 10 minutes) isperformed such that the Mg alloy layer is in contact with the m-planesurface of the p-type GaN layer, Ga atoms in the p-type GaN layer arediffused toward the electrode side, whereas N atoms are scarcelydiffused toward the electrode side. As a result, the outermost surfaceof the p-type GaN layer is lacking Ga atoms, so that Ga vacancies areformed. The Ga vacancies have accepter-like properties, and therefore,as the number of Ga vacancies increases in the vicinity of the interfacebetween the electrode and the p-type GaN layer, holes more readily passthrough the Schottky barrier of this interface by means of tunneling.Thus, it is estimated that the contact resistance would decrease whenthe Mg alloy layer is formed so as to be in contact with the m-planesurface of the p-type GaN layer.

On the other hand, the present inventor found that, when the heattreatment (at 600° C. in a nitrogen atmosphere for 10 minutes) isperformed such that the Mg alloy layer is in contact with the c-planesurface, instead of the m-plane surface, of the p-type GaN layer, Natoms as well as Ga atoms are diffused toward the electrode side. Also,the present inventor confirmed that, in this case, the contactresistance is high. As N atoms as well as Ga atoms are diffused towardthe electrode side, N vacancies that have donor-like properties are alsoformed in the outermost surface of the p-type GaN layer. As a result, atthe outermost surface of the p-type GaN layer, charge compensationoccurs between the Ga vacancies and the N vacancies. It is estimatedthat the omission of the N atoms would degrade the crystallinity of GaNcrystals. It is inferred from such reasons that, when the Mg alloy layeris formed so as to be in contact with the c-plane surface of GaN, thecontact resistance is high.

This finding teaches that the physical properties, such as interatomicbond strength, surface condition, etc., are totally different betweenthe m-plane GaN and the c-plane GaN.

It is inferred that the behaviors of respective ones of such elements(Ga, N) occur even when some of Ga atoms are replaced by Al or In atomsin the GaN layer with which the Mg alloy layer is in contact. It is alsoinferred that the same applies even when the GaN-based semiconductorlayer with which the Mg alloy layer is in contact is doped with anelement other than Mg as a dopant.

Next, referring again to FIG. 3( a), the structure of the presentembodiment is described in more detail.

As shown in FIG. 3( a), the light-emitting device 100 of the presentembodiment includes the m-plane GaN substrate 10 and theAl_(u)Ga_(v)In_(w)N layer 22 (where u+v+w=1, u≧0, v≧0, w≧0) provided onthe substrate 10. In this example, the m-plane GaN substrate 10 is ann-type GaN substrate (for example, 100 μm thick). TheAl_(u)Ga_(v)In_(w)N layer 22 is an n-type GaN layer (for example, 2 μmthick). Provided on the Al_(u)Ga_(v)In_(w)N layer 22 is an active layer24. In other words, a semiconductor multilayer structure 20 including atleast the active layer 24 is provided on the m-plane GaN substrate 10.

In the semiconductor multilayer structure 20, an active layer 24including an Al_(a)In_(b)Ga_(c)N layer (where a+b+c=1, a≧0, b≧0 and c≧0)has been formed on the Al_(x)Ga_(y)In_(z)N layer 22. The active layer 24consists of InGaN well layers with an In mole fraction of approximately25% and GaN barrier layers, both the well layers and the barrier layersmay have a thickness of 9 nm each, and the well layers may have a welllayer period of three. On the active layer 24, stacked is anAl_(d)Ga_(e)N layer (where d+e=1, d≧0 and e≧0) 26 of the secondconductivity type (which may be p-type, for example), which may be anAlGaN layer with an Al mole fraction of 10% and may have a thickness of0.2 μm. In this preferred embodiment, the Al_(d)Ga_(e)N layer 26 isdoped with Mg as a p-type dopant to a level of approximately 10¹⁸ cm⁻³,for example. Also, in this example, an undoped GaN layer (not shown) isinterposed between the active layer 24 and the Al_(d)Ga_(e)N layer 26.

Furthermore, in this example, on the Al_(d)Ga_(e)N layer 26, stacked isa GaN layer (not shown) of the second conductivity type (which may bep-type, for example). In addition, on the contact layer of p⁺-GaN,stacked in this order are an Mg alloy layer 32 and a Pt layer 34. Andthis stack of the Mg alloy layer 32 and the Pt layer 34 is used as anelectrode (i.e., a p-electrode) 30.

This semiconductor multilayer structure 20 further has a recess 42 thatexposes the surface of the Al_(u)Ga_(v)In_(w)N layer 22. And anelectrode 40 (n-electrode) has been formed on the Al_(u)Ga_(v)In_(w)Nlayer 22 at the bottom of the recess 42, which may have a width (ordiameter) of 20 μm and a depth of 1 μm, for example. The electrode 40may have a multilayer structure consisting of Ti, Al and Pt layers,which may have thicknesses of 5 nm, 100 nm and 10 nm, respectively.

The present inventors discovered that the nitride-based semiconductorlight-emitting device 100 of this preferred embodiment could have anoperating voltage Vop that was approximately 1.3 V lower than that of aconventional m-plane LED with a Pd/Pt electrode, and therefore, couldcut down the power dissipation as a result.

Next, a method for fabricating the nitride-based semiconductorlight-emitting device 100 of this embodiment is described while stillreferring to FIG. 3( a).

First of all, an m-plane substrate 10 is prepared. In this preferredembodiment, a GaN substrate is used as the substrate 10. The GaNsubstrate of this preferred embodiment is obtained by HVPE (hydridevapor phase epitaxy).

For example, a thick GaN film is grown to a thickness of severalmillimeters on a c-plane sapphire substrate, and then dicedperpendicularly to the c-plane (i.e., parallel to the m-plane), therebyobtaining m-plane GaN substrates. However, the GaN substrate does nothave to be prepared by this particular method. Alternatively, an ingotof bulk GaN may be made by a liquid phase growth process such as asodium flux process or a melt-growth method such as an ammonothermalprocess and then diced parallel to the m-plane.

The substrate 10 does not have to be a GaN substrate but may also be agallium oxide substrate, an SiC substrate, an Si substrate or a sapphiresubstrate, for example. To grow an m-plane GaN-based semiconductor onthe substrate by epitaxy, the principal surface of the SiC or sapphiresubstrate is preferably also an m-plane. However, in some instances,a-plane GaN could grow on an r-plane sapphire substrate. That is whyaccording to the growth conditions, the surface on which the crystalgrowth should take place does not always have to be an m-plane. In anycase, at least the surface of the semiconductor multilayer structure 20should be an m-plane. In this preferred embodiment, crystal layers areformed one after another on the substrate 10 by MOCVD (metalorganicchemical vapor deposition) process.

Next, an Al_(u)Ga_(v)In_(w)N layer 22 is formed on the m-plane GaNsubstrate 10. As the Al_(u)Ga_(v)In_(w)N layer 22, AlGaN may bedeposited to a thickness of 3 μm, for example. A GaN layer may bedeposited by supplying TMG(Ga(CH₃)₃), TMA(Al(CH₃)₃) and NH₃ gases ontothe m-plane GaN substrate 10 at 1,100° C., for example.

Subsequently, an active layer 24 is formed on the Al_(u)Ga_(v)In_(w)Nlayer 22. In this example, the active layer 24 has a GaInN/GaNmulti-quantum well (MQW) structure in which Ga_(0.9)In_(0.1)N welllayers and GaN barrier layers, each having a thickness of 9 nm, havebeen stacked alternately to have an overall thickness of 81 nm. When theGa_(0.9)In_(0.1)N well layers are formed, the growth temperature ispreferably lowered to 800° C. to introduce In.

Thereafter, an undoped GaN layer is deposited to a thickness of 30 nm,for example, on the active layer 24, and then an Al_(d)Ga_(e)N layer 26is formed on the undoped GaN layer. As the Al_(d)Ga_(e)N layer 26,p-Al_(0.14)Ga_(0.86)N is deposited to a thickness of 70 nm by supplyingTMG, NH₃, TMA, TMI gases and Cp₂Mg (cyclopentadienyl magnesium) gas as ap-type dopant.

Next, a p-GaN contact layer is deposited to a thickness of 0.5 μm, forexample, on the Al_(d)Ga_(e)N layer 26. In forming the p-GaN contactlayer, Cp₂Mg is supplied as a p-type dopant.

Thereafter, respective portions of the p-GaN contact layer, theAl_(d)Ga_(e)N layer 26, the undoped GaN layer, and the active layer 24are removed by performing a chlorine-based dry etching process, therebymaking a recess 42 and exposing a region of the Al_(x)Ga_(y)In_(z)Nlayer 22 where an n-electrode will be formed. Then, Ti/Pt layers aredeposited as an n-electrode 40 on the region reserved for an n-typeelectrode at the bottom of the recess 42.

Then, an Mg layer (thickness: 2 nm) is formed on the p-GaN contactlayer, and a Pt layer (thickness: 75 nm) is formed on the Mg layer.Thereafter, the resultant structure is subjected to a heat treatment at600° C. in a nitrogen atmosphere for 10 minutes, so that part of the Ptlayer which is on the Mg layer side encroaches upon the Mg layer,thereby forming the Mg alloy layer 32. The other part of the Pt layerwhich has not been alloyed with the Mg layer is remaining as the Ptlayer 34 on the Mg alloy layer 32. The heat treatment of the presentembodiment serves as both the heat treatment for formation of the Mgalloy layer and the heat treatment for diffusing Ga atoms of the p-typeGaN layer toward the electrode side.

The present embodiment uses, for formation of the Mg layer, a pulseevaporation process in which deposition is performed while a materialmetal is evaporated in pulses. More specifically, metal Mg contained ina crucible in a vacuum is irradiated with pulses of electron beam,whereby the material metal is evaporated in pulses. Some of themolecules or atoms of that material metal are deposited on the p-GaNcontact layer, whereby an Mg layer is formed. For example, those pulsesmay have a pulse width of 0.5 seconds and may be applied repeatedly at afrequency of 1 Hz. By adopting such a method, a dense film of highquality could be formed as the Mg layer. The Mg layer had such highdensity probably because, by performing such a pulse evaporationprocess, Mg atoms or a cluster of Mg atoms that collide against thep-GaN contact layer would have their kinetic energy increased.

Generally speaking, Mg is an element which is susceptible to oxidationwhen exposed to water or air. However, an Mg layer obtained by using thepulse evaporation process of the present embodiment is resistant tooxidation and exhibits excellent water resistance and oxygen resistance.

This preferred embodiment uses a technique for depositing a layer whileevaporating the material metal (i.e., metal Mg) in pulses. However, aslong as the Mg layer can be formed, any other technique can also beadopted. As an alternative method for forming such a dense Mg layer ofquality, sputtering, a thermal CVD process, or a molecular beam epitaxy(MBE) could also be used.

Optionally, the substrate 10 and a portion of the Al_(u)Ga_(v)In_(w)Nlayer 22 could be removed after that by some technique such as laserlift-off, etching or polishing. In that case, either only the substrate10 or the substrate 10 and a portion of the Al_(u)Ga_(v)In_(w)N layer 22could be removed selectively. It is naturally possible to leave thesubstrate 10 and the Al_(u)Ga_(y)In_(w)N layer 22 as they are withoutremoving them. By performing these process steps, the nitride-basedsemiconductor light-emitting device 100 of this preferred embodiment iscompleted.

In the nitride-based semiconductor light-emitting device 100 of thispreferred embodiment, when a voltage is applied to between the n- andp-electrodes 40 and 30, holes are injected from the p-electrode 30 intothe active layer 24 and electrons are injected from the n-electrode 40into the active layer 24, thus producing photoluminescence with awavelength of about 450 nm.

FIG. 8( a) shows the current-voltage characteristic of a light-emittingdiode that uses an electrode consisting of Mg—Pt alloy/Pt layers onm-plane GaN. For comparison purposes, the characteristics oflight-emitting diodes (conventional examples) that have the same LEDnitride-based semiconductor structure but that use an electrodeconsisting of Pd/Pt layers and an electrode consisting of Mg/Pt layers,respectively, are also shown in FIG. 8( a). The electrode structures andthe heat treatment conditions for these light-emitting diodes are shownin TABLE 3.

TABLE 3 Heat treatment Plane Thickness (before temperature orientationp-electrode heat treatment) and duration m-plane Mg—Pt Alloy/Pt Mg 2nm/Pt 75 nm 600° C., 10 min. m-plane Mg/Pt Mg 7 nm/Pt 75 nm 600° C., 10min. m-plane Pd/Pt Pd 40 nm/Pt 35 nm 500° C., 10 min.

In each of these light-emitting diodes, an n-type GaN layer, an activelayer in which three InGaN well layers and two GaN barrier layers arealternately stacked one upon the other, and a p-type GaN layer arestacked in this order on an m-plane GaN substrate. In addition, eitheran Mg/Pt electrode or a Pd/Pt electrode is provided as a p-electrode onthe p-type GaN layer. On the other hand, an n-electrode is formed on then-type GaN layer by etching the p-type GaN layer and the active layerand exposing the n-type GaN layer.

First, a conventional electrode (an electrode of Pd/Pt layers) and anelectrode of the present embodiment (an electrode of Mg—Pt alloy/Ptlayers) are compared. The threshold voltage of a light-emitting diodewhich includes the electrode of Pd/Pt layers is about 3.2 V, whereas thethreshold voltage of a light-emitting diode which includes the electrodeof Mg—Pt alloy/Pt layers is about 2.7 V. That is, the threshold voltageof the present embodiment is smaller than that of the conventionaldevice. It is understood from the comparison in terms of the operatingvoltage with the current value of 20 mA that the operating voltage ofthe light-emitting diode which includes the electrode of Mg—Pt alloy/Ptlayers is lower than that of the light-emitting diode which includes theelectrode of Pd/Pt layers by 1.3 V or more. Thus, the light-emittingdiode which includes the electrode of the present embodiment is capableof greatly reducing the operating voltage as compared with theconventional diode.

Next, comparing the electrode of the present embodiment (the electrodeof Mg—Pt alloy/Pt layers) and the electrode of Mg/Pt layers, thethreshold voltage and the operating voltage for the current value of 20mA of the light-emitting diode which includes the electrode of thepresent embodiment are somewhat larger than those of a light-emittingdiode which includes the electrode of Mg/Pt layers.

FIG. 8( b) is a graph showing the contact resistances of the Mg—Ptalloy/Pt electrode, the Pd/Pt electrode, and the Mg/Pt electrode forcomparison purposes. In any sample, the electrode is in contact with thep-type GaN layer.

The thicknesses of the respective layers before the heat treatment areshown below in TABLE 4.

TABLE 4 Plane Thickness (before orientation p-electrode heat treatment)m-plane Mg—Pt Alloy/Pt Mg 2 nm/Pt 75 nm m-plane Pd/Pt Pd 40 nm/Pt 35 nmm-plane Mg/Pt Mg 7 nm/Pt 75 nm

The temperature and duration of the heat treatment are shown below inTABLE 5.

TABLE 5 Heat treatment Plane temperature and orientation p-electrodeduration m-plane Mg—Pt Alloy/Pt 600° C., 10 min. m-plane Pd/Pt 500° C.,10 min. m-plane Mg/Pt 600° C., 10 min.

As seen from FIG. 8( b), the contact resistance of the electrode ofMg—Pt alloy/Pt layers is lower than that of the electrode of Pd/Ptlayers. The contact resistance of the electrode of Mg—Pt alloy/Pt layersis somewhat higher than that of the electrode of Mg/Pt layers.

As seen from the results shown in FIGS. 8( a) and 8(b), the electriccharacteristics (the characteristics of the threshold voltage and theoperating voltage) and the contact resistance of the electrode of thepresent embodiment are somewhat inferior to those of the electrode ofMg/Pt layers. However, in terms of adhesiveness, the electrode of thepresent embodiment exhibits a superior characteristic to that of theelectrode of Mg/Pt layers. Hence, it can be said that the electrode ofthe present embodiment is superior in terms of reliability.

FIG. 9( a) is a light microscopic photograph of an electrode surface ofa light-emitting device which includes the electrode of Mg—Pt alloy/Ptlayers, which is presented as a substitute for a drawing. FIG. 9( b) isa light microscopic photograph of an electrode surface of alight-emitting device which includes the electrode of Mg/Pt layers,which is presented as a substitute for a drawing. In the light-emittingdevice which includes the electrode of Mg—Pt alloy/Pt layers, peelingoff of the p-electrode 30 did not occur as shown in FIG. 9( a). In thelight-emitting device which includes the electrode of Mg/Pt layers,peeling off was detected in part of an edge of the p-electrode 130 asshown in FIG. 9( b) in some samples. Note that FIG. 9( b) is aphotograph of one of fabricated light-emitting device samples in whichpeeling off of the electrode was detected. It is not suggested that suchpeeling off of the electrode may occur with high probability in thedevices which include the electrode of Mg/Pt layers.

Next, examples that used an electrode consisting of an Au layer and anelectrode consisting of Mg—Au alloy/Au layers will be described ascomparative examples with reference to FIG. 10. Specifically, FIG. 10(a) shows the measured specific contact resistances (Ω·cm²) of such anelectrode consisting of an Au layer and such an electrode consisting ofMg—Au alloy/Au layers that were formed on an m-plane GaN layer. Itshould be noted that these specific contact resistances were measuredafter the electrode had been formed and thermally treated. The electrodeof the Mg—Au alloy/Au layers was formed by forming a layered structureof an Mg layer and an Au layer and thereafter performing a heattreatment on the layered structure at 600° C. for 10 minutes. Since Mgand Au can readily be alloyed by a heat treatment, it is estimated thatthe Mg layer and the Au layer would be changed by the heat treatmentinto a layered structure of an Mg—Au alloy layer and an Au layer (i.e.,Mg—Au alloy/Au layers).

As seen from the result of FIG. 10( a), the specific contact resistancecharacteristic is lower when the electrode of Mg—Au alloy/Au layers isused than when the electrode of an Au layer is used. Note that it wasconfirmed that the contact resistance of the electrode of the Au layerwas substantially equal to that of the electrode of Pd/Pt layers. In isunderstood from the result of FIG. 10( a) that the electrode of Mg—Aualloy/Au layers exhibits a higher contact resistance than the electrodeof Pd/Pt layers. In this respect, it is significantly different from theresult, i.e., improved characteristic, of the electrode of the presentembodiment (e.g., Mg—Pt alloy/Pt layers). Note that, as described above,Mg is an element which is susceptible to oxidation when exposed to wateror air, and therefore, a multilayer configuration with an Au layer(Mg—Au alloy/Au layers after the heat treatment), rather than anelectrode simply consisting of an Mg layer, can be one proposedstructure. However, in actuality, the Mg—Au alloy/Au layers lead to ahigher contact resistance than the Au layer and therefore lead to poorcontact characteristics. In other words, the excellent contactresistance characteristic achieved by the configuration of the presentembodiment (e.g., Mg—Pt alloy/Pt layers) would not have been anticipatedby a person skilled in the art in view of the unfavorable measurementresult of the electrode consisting of Mg—Au alloy and Au layers.

Note that, in the results shown in FIG. 10( a), in the Au electrode (orPd/Pt electrode), the absolute value of the contract resistance isrelatively low (not more than 3×10⁻³ Ω·cm²). This is because, in them-plane GaN layer used in this experiment, the amount of implanted Mgdopant is optimized. However, measuring the current-voltagecharacteristic while two Au electrodes (or Pd/Pt electrodes) were incontact with the p-type GaN layer resulted in detection of a Schottkyvoltage. Thus, Au is not favorable for a material of an electrode whichis to be in contact with the m-plane surface of the p-type GaN layer. Onthe other hand, the m-plane GaN layer was used to fabricate theelectrode of the present embodiment (e.g., Mg—Pt alloy/Pt), and thecontact resistance measured was 5×10⁻⁴ Ω·cm² or less. Also, theelectrode of the present embodiment was measured as to thecurrent-voltage characteristic while the electrode was in contact withthe p-type GaN layer, but a Schottky voltage was not detected. It wasthus found that the electrode of the present embodiment and the p-typeGaN layer with the m-plane surface formed an ohmic contact.

FIG. 10( b) is a photograph of a surface of the electrode of Mg—Aualloy/Au layers after the heat treatment, which is presented as asubstitute for a drawing. On the other hand, FIG. 10( c) is a photographof a surface of the electrode of the Au layer after the heat treatment,which is presented as a substitute for a drawing. Comparing thesephotographs, it was found that the electrode of Mg—Au alloy/Au layershad an inferior film quality.

While the present invention has been described with respect to preferredembodiments thereof, this invention is in no way limited to thosespecific preferred embodiments but could be modified in numerous waysand may assume many embodiments other than those specifically describedabove.

Even though its structure is essentially different from the preferredembodiment of the present invention, related structures are alsodisclosed in Patent Documents 3 and 4. However, those Patent Documents 3and 4 do not mention at all that the crystallographic plane of theirgallium nitride-based semiconductor layer is an m-plane but justdisclose a technique for forming an electrode on a c-plane galliumnitride-based semiconductor layer. More specifically, Patent Document 3discloses a structure in which an Au layer is stacked on an Mg layer.And even if an electrode with such a multilayer structure were formed onan m-plane, the effect of the electrode of this preferred embodimentwould never be achieved. Meanwhile, Patent Document 4 mentions metallayers of Ni, Cr and Mg but discloses only a specific example about anelectrode structure that uses an Ni layer as the lower layer. Both ofthese Patent Documents 3 and 4 relate to an electrode structure that hasbeen formed on a c-plane gallium nitride-based semiconductor layer andteach neither a problem nor a solution about the contact resistance withrespect to an m-plane gallium nitride-based semiconductor layer.

The present inventor also disclosed, in the earlier application(Japanese Patent Application No. 2009-030147), that an electrodeconfiguration in which an Mg layer is in contact with an m-plane surfaceof a p-type GaN layer (Mg electrode) exhibits a low contact resistance.The contact resistance of the Mg alloy layer electrode of the presentinvention is higher than that of the electrode disclosed in the earlierapplication. However, as illustrated in FIG. 8( a), the effect ofdecreasing the operating voltage of a light-emitting diode whichincludes the Mg alloy layer electrode of the present invention isremarkably superior to that of the conventional Pd/Pt electrode. Also,the Mg alloy layer has a higher adhesiveness with the semiconductormultilayer structure than the Mg layer does and is thereforeadvantageous in improving the yields in mass production processes andimproving the device reliability than the Mg electrode. There areseveral possible reasons for the high adhesiveness between the Mg alloylayer and the semiconductor multilayer structure. Addition of Pt (or Mo,Pd) to Mg increases the resistance to oxidation. Improvement in rigiditydecreases the warpage due to strain. Pt (or Mo, Pd) contained in the Mgalloy layer is in contact with the semiconductor multilayer structure sothat the adhesiveness is improved as compared with an electrodeconsisting of Mg only.

The light-emitting device of the present invention described above couldbe used as it is as a light source. However, if the light-emittingdevice of the present invention is combined with a resin including aphosphor that produces wavelength conversion, for example, the device ofthe present invention can be used effectively as a light source with anexpanded operating wavelength range (such as a white light source).

FIG. 11 is a schematic representation illustrating an example of such awhite light source. The light source shown in FIG. 11 includes alight-emitting device 100 with the structure shown in FIG. 3( a) and aresin layer 200 in which particles of a phosphor such as YAG (YttriumAluminum Garnet) are dispersed to change the wavelength of the lightemitted from the light-emitting device 100 into a longer one. Thelight-emitting device 100 is mounted on a supporting member 220 on whicha wiring pattern has been formed. And on the supporting member 220, areflective member 240 is arranged so as to surround the light-emittingdevice 100. The resin layer 200 has been formed so as to cover thelight-emitting device 100.

Note that the contact structure of the present invention provides theabove-described excellent effects when the p-type semiconductor regionthat is in contact with the Mg layer is formed of a GaN-basedsemiconductor, specifically an Al_(x)In_(y)Ga_(z)N semiconductor(x+y+z=1, x≧0, y≧0, z≧0). As a matter of course, such an effect ofreducing the contact resistance can also be obtained in light-emittingdevices other than LEDs (e.g., semiconductor lasers) and devices otherthan the light-emitting devices (e.g., transistors and photodetectors).

The actual m-plane does not always have to be a plane that is exactlyparallel to an m-plane but may be slightly tilted from the m-plane by0±1 degree.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to reduce the contactresistance of a GaN-based semiconductor device fabricated by producingcrystal growth on an m-plane substrate or a GaN-based semiconductormultilayer structure, of which the principal surface is an m-plane. As aresult, although such a GaN-based semiconductor device fabricated byproducing crystal growth on an m-plane substrate (or a GaN-basedsemiconductor multilayer structure, of which the principal surface is anm-plane) has been difficult to use extensively owing to its bad contactresistance characteristic, its industrial applicability can be expandedsignificantly by the present invention.

REFERENCE SIGNS LIST

10 substrate (GaN-based substrate)

12 surface of substrate (m-plane)

20 semiconductor multilayer structure

22 Al_(u)Ga_(v)In_(w)N layer

24 active layer

26 Al_(d)Ga_(e)N layer

30 p-electrode

32 Mg alloy layer

34 metal layer (Pt layer)

40 n-electrode

42 recess

100 nitride-based semiconductor light-emitting device

200 resin layer

220 supporting member

240 reflective member

1. A nitride-based semiconductor device, comprising: a nitride-basedsemiconductor multilayer structure including a p-type semiconductorregion, a surface of the p-type semiconductor region being an m-plane;and an electrode that is arranged on the p-type semiconductor region,wherein the p-type semiconductor region is made of anAl_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg alloy layer which is in contact withthe surface of the p-type semiconductor region and which is made of Mgand a metal selected from a group consisting of Pt, Mo, and Pd.
 2. Thenitride-based semiconductor device of claim 1, wherein the electrodeincludes the Mg alloy layer and a metal layer provided on the Mg alloylayer, and the metal layer is formed of a metal which is selected fromPt, Mo, and Pd and which is contained in the Mg alloy layer.
 3. Thenitride-based semiconductor device of claim 1, wherein the semiconductormultilayer structure includes an active layer which includes anAl_(a)In_(b)Ga_(c)N layer (where a+b+c=1, a≧0, b≧0 and c≧0), the activelayer being configured to emit light.
 4. The nitride-based semiconductordevice of claim 1, wherein the p-type semiconductor region is a p-typecontact layer.
 5. The nitride-based semiconductor device of any one ofclaims 1 to 4, wherein the Mg alloy layer has a thickness of 0.1 nm to 5nm.
 6. The nitride-based semiconductor device of claim 2, wherein thethickness of the Mg alloy layer is equal to or smaller than that of themetal layer.
 7. The nitride-based semiconductor device of claim 1wherein, in the Mg alloy layer, a concentration of N is lower than aconcentration of Ga.
 8. The nitride-based semiconductor device of claim1, further comprising a semiconductor substrate that supports thesemiconductor multilayer structure.
 9. A light source, comprising: anitride-based semiconductor light-emitting device; and a wavelengthconverter including a phosphor that converts a wavelength of lightemitted from the nitride-based semiconductor light-emitting device,wherein the nitride-based semiconductor light-emitting device includes anitride-based semiconductor multilayer structure including a p-typesemiconductor region, a surface of the p-type semiconductor region beingan m-plane, and an electrode that is arranged on the p-typesemiconductor region, the p-type semiconductor region is made of anAl_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg alloy layer which is in contact withthe surface of the p-type semiconductor region and which is made of Mgand a metal selected from a group consisting of Pt, Mo, and Pd.
 10. Amethod for fabricating a nitride-based semiconductor device, comprisingthe steps of: (a) providing a substrate; (b) forming on the substrate anitride-based semiconductor multilayer structure including a p-typesemiconductor region, a surface of the p-type semiconductor region beingan m-plane; and (c) forming an electrode on the surface of the p-typesemiconductor region of the semiconductor multilayer structure, whereinstep (c) includes forming an Mg alloy layer on the surface of the p-typesemiconductor region, the Mg alloy layer being made of Mg and a metalselected from a group consisting of Pt, Mo, and Pd.
 11. The method ofclaim 10, wherein the step of forming the Mg alloy layer includesforming an Mg layer on the surface of the p-type semiconductor region,forming on the Mg layer a conductive layer selected from a groupconsisting of Pt, Mo, and Pd, and performing a heat treatment to alloythe Mg layer and at least part of the conductive layer.
 12. The methodof claim 11, wherein the heat treatment is performed at a temperature of500° C. to 700° C.
 13. The method of claim 12, wherein the heattreatment is performed at a temperature of 550° C. to 650° C.
 14. Themethod of claim 11, wherein the step of forming the Mg layer includesirradiating Mg with pulses of an electron beam such that Mg is depositedonto the surface of the p-type semiconductor region.
 15. The method ofclaim 11, wherein the Mg layer is deposited on the semiconductormultilayer structure so as to have a thickness of 0.1 nm to 5 nm. 16.The method of claim 10, further comprising removing the substrate afterstep (b).
 17. The method of claim 10, wherein the step of forming the Mgalloy layer includes depositing a mixture or compound of Mg and a metalselected from a group consisting of Pt, Mo, and Pd onto the surface ofthe p-type semiconductor region by means of evaporation, and performinga heat treatment.
 18. A nitride-based semiconductor device, comprising:a nitride-based semiconductor multilayer structure including a p-typesemiconductor region, a surface of the p-type semiconductor region beingan m-plane; and an electrode that is arranged on the p-typesemiconductor region, wherein the p-type semiconductor region is made ofan Al_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg alloy island provided on the surface ofthe p-type semiconductor region, the Mg alloy island being made of Mgand a metal selected from a group consisting of Pt, Mo, and Pd.
 19. Thenitride-based semiconductor device of claim 18, wherein the electrodeincludes the Mg alloy island and a metal layer provided on the Mg alloyisland, and the metal layer is formed of a metal which is selected fromPt, Mo, and Pd and which is contained in the Mg alloy island.